The present invention relates to photonic crystals and methods of making photonic crystals. More specifically, the present invention relates to self assembled photonic crystals having opal structures.
Photonic crystals with a photonic bandgap are expected to be used in the variety of applications including optical filters, sharp bending light guides, and very low threshold lasers, and have been investigated widely. Of special interest are three-dimensional photonic crystals, which may have lower loss than two-dimensional photonic crystals of the slab type (see S. Noda, K. Tomoda, N. Yamanoto, A. Chutinan, “Full three-dimensional photonic bandgap crystals at near-infrared wavelengths”, Science, vol. 289, pp. 604–606, July, 2000). In general, the fabrication processes of photonic crystals require the use of complicated and expensive semiconductor nanofabrication techniques. An alternative fabrication method for photonic crystals takes the form of self assembled crystalline structures using colloidal spheres (see, Y. A. Vlasov, X.-Z. Bo, J. C. Sturm, D. J. Norris, “On-chip natural assembly of silicon photonic bandgap crystals”, Nature, vol. 414, pp. 289–293, November, 2001). The crystalline structure opal has a hexagonal closed-packed or face centered cubic lattice. The opal is removed after infiltration with high refractive index material to obtain the inverse opal, which has a complete three-dimensional photonic bandgap. Inverse opals with a defect can be expected to produce highly efficient filters and low threshold lasers. Therefore, high quality crystalline opals are required to make the inverse opals. For high quality crystalline opals, first, silica spheres should have a required diameter with narrow size distribution, which determines the wavelength of the photonic bandgap (or the maximum reflectance peak on the optical spectrum). Second, during the formation process, aggregation and adhesions between spheres, which results in “doublets” must be avoided. Such formations result in structural defects and dislocations in the final crystal.